1. Field of the Invention
This invention relates to a transmitter for use in a radio system operable according to a non-constant envelope modulation scheme—particularly the Wideband Code Division Multiple Access (WCDMA) standard. It finds particular, but not exclusive utility in a radio system employing both Time Division Duplex (TDD) and Frequency Division Duplex (FDD). However, it also provides benefits in a system employing only one of these, or other, technologies.
2. Description of the Prior Art
WCDMA technology is being used to provide access to third generation (3G) mobile telecommunication systems. Two particular variants of WCDMA are TDD and FDD. FDD is intended to be used for macrocellular applications, and splits the transmit and receive bands into two distinct frequency spectra. A possible application for TDD is in micro- and pico-cellular applications. TDD splits the transmit and receive channels on a timeslot basis—each residing on the same physical channel. TDD is better suited to applications requiring different bandwidths in the uplink and downlink directions, as the amount of bandwidth allocated in each direction can be dynamically varied. In this way, a user who is web browsing, for instance, can have more timeslots allocated to the downlink, while receiving more data than being sent.
The standards and regulations governing devices operating according to WCDMA specify certain key parameter ranges within which all compliant devices must operate. One key signal quality metric is Adjacent Channel Leakage Ratio (ACLR). This metric gives a measure of the interference experienced on a channel adjacent to that currently being used for transmission. A primary cause of poor ACLR performance is operation of the transmitter's power amplifier (PA) in a non-linear region. Use of a PA having an insufficient dynamic range can result in the output signal from the PA being compressed . Such compression will spread the output spectrum of the PA. This spreading can leak through into adjacent channels, resulting in poor ACLR performance.
Prior art PAs address this problem by specifying the PA to operate in a linear region for all output powers of the PA. Achieving linear performance from PAs requires them to be operated in an inefficient manner i.e. the ratio of RF Power out, to DC power in to the PA, is poor. This arrangement, which has thus far been required in order to comply with the operating standards, has resulted in PAs which require high currents from the device power supply. For mobile devices, this consequently leads to poor battery life, which has an impact on talk time. In the case of base stations, the operating costs associated with inefficient power supplies can be sizeable.
WCDMA signals typically have a high peak to average ratio (PAR). This means that the peak signal from the PA can sometimes be several dB higher than the average signal. In prior art transmitters, the PA must be operable in a linear fashion across this entire range. This results in inefficiencies as described previously. The highest value of PAR experienced in a system is defined as the ‘crest factor’.
It is desirable to reduce the crest factor, and thus the difference between the peak and the average output power from the PA.